The characterization and testing of magnetic bubble memories plays an important role in all phases of magnetic bubble memory manufacturing. Commercially available magnetic bubble memories operate in a three dimensional magnetic field. The first field referred to as bias or Z field is required for the development and maintenance of the bubbles within the magnetic material. If the bias field is too strong, the bubbles will collapse. If the bias field is too weak, the bubbles will turn into a serpentine shaped domain. Therefore, the bias field must be operated at a level consistent with good bubble maintenance. This difference between the high field and low field is referred to as the bias margin and a good bias margin is required to operate a bubble memory device effectively. Additionally, a holding bias vector is required in the X-Y plane to prevent bubble migration during the standby or power down mode. This is accomplished either by a slight tilt of the chip in the Z field, or by the addition of holding bias coils to provide the required field.
The X and Y fields, respectively, are developed by applying an alternating current source to an X coil and a phase differentiated alternating current to a Y coil in such a manner so as to induce a magnetic field in the plane of the tested magnetic bubble memory device and as the frequency modulates the induced magnetic field of the X and Y coils the resultant magnetic vector in the plane of the test chip rotates at one revolution for each period of the applied signal to the coil. When the chip is at the probe stage or the first check of the slice itself, it is common practice to apply a sinusoidal drive current to both the X and Y coils primarily due to the large currents and voltages required since the coils must be large and located at some greater distance from the chip than is required in actual operation. Since these currents must be large to develop a uniform magnetic field across the surface of the chip a sinusoid is easier to maintain and is easier to control with respect to the changing magnetic fields and large currents required.
Once the device has been fabricated, however, many magnetic bubble memory users drive the chip utilizing a triangular or sawtooth wave because of its relative simplicity to generate on the magnetic bubble memory board and the smaller currents required in its final assembled form. This results in a discrepancy between those devices which pass probe operated in a sinusoid drive but will fail final tests operated at a triangular drive primarily due to the differences between the margins and the rotating vector as plotted on a X Y graph forming different shapes. A sinusoid will form a circle where a combination of triangular waves applied to the X and Y coils will form a diamond shape. This diamond shape may propagate without any difficulty. However, the replication, swap and other functions may not operate as well and therefore cause a failure at final test for an otherwise acceptable chip. Further compounding this problem is the high noise level of the probe environment requiring several techniques to eliminate as much background noise as possible. For example, see the Roman Kowalchuk article entitled "Bubble Memory Testing at Western Electric" at pages 70 and 71, 1979, IEEE Test Conference, Cherry Hill, N.J. Also see the David C. Chang and John E. Davies article entitled "Characterization and Testing of Magnetic Bubble Memories" also 1979, IEEE Test Conference, Cherry Hill, N.J., and see Steve Bisset, entitled "Development and Execution of Bubble Memory Test Sequences", 1979, IEEE Test Conference, Cherry Hill, N.J.
Accordingly, it is an objective of the present invention to provide a magnetic field waveform control apparatus capable of producing desired magnetic fields in one or more spatial axes closely corresponding to a desired waveform or set of waveforms.
It is a further objective of the present invention to provide an engineering tool by which varying electrical and magnetic waveforms can be applied to a device to test mathematical predictions of dynamic characteristics of magnetic bubble cell designs, propagation parameters, replication and other necessary circuit functions, as well as materials and fabrication technique variations with respect to operating margins and waveform requirements.
Another objective of the present invention is to provide a flexible probe tester for in depth analysis of each bubble chip with respect to closely simulated operating conditions to avoid unnecessary expenditures on marginal chips.
Another objective of the present invention is to provide a final product testing device for analyzing end product margins, as well as an advanced degree of quality and reliability control over the final product.
Another objective of the present invention is to provide a magnetic bubble memory device tester for use in incoming device testing as well as checking device parameters after their sale to provide a trouble shooting tool and device tester for original equipment manufacturers of memory equipment using magnetic bubble memories.